An electronic image generator, such as television set, scans a viewable image, or a sequence of viewable video images, onto a display screen by electronically sweeping an electromagnetic image beam across the screen. For example, in a television set, the image beam is a beam of electrons, and a coil generates a linearly increasing magnetic field or electric field to sweep the beam.
An optical image generator is similar except that it scans a viewable image onto a display screen by mechanically sweeping an electromagnetic image beam across the screen. Or, in the case of a Virtual Retinal Display (VRD), the optical image generator scans a viewable directly onto a viewer's retina(s).
FIG. 1 is a view of a conventional optical image-display system 10, which includes an optical image generator 12 and a display screen 14. The image generator 12 includes a beam generator 16 for generating an optical beam 18, and includes a scan assembly 20 for scanning an image onto the screen 14 with the beam. Where the system 10 is a VRD, the scan assembly 20 scans the image directly onto a viewer's retina(s) (not shown). The scan assembly 20 includes a reflector 22, which simultaneously rotates back and forth in the horizontal (X) and vertical (Y) dimensions about pivot arms 24a and 24b and pivot arms 26a and 26b; respectively. By rotating back and forth, the reflector 22 sweeps the beam 18 in a two-dimensional (X-Y) raster pattern to generate the image on the screen 14 (or retina(s)). The scan assembly 20 includes other components and circuitry (not shown) for rotating the reflector 22 and monitoring its instantaneous rotational position, which is proportional to the instantaneous location at which the beam 18 strikes the screen 14. In an alternative implementation that is not shown, the scan assembly 20 may include two reflectors, one for sweeping the beam 18 in the horizontal (X) dimension and the other for sweeping the beam in the vertical (Y) dimension. An optical image-display system that is similar to the system 10 is disclosed in U.S. Pat. No. 6,140,979 of Gerhard, et al., entitled SCANNED DISPLAY WITH PINCH, TIMING, AND DISTORTION CORRECTION and U.S. Pat. No. 5,467,104 of Furness, et al., entitled VIRTUAL RETINAL DISPLAY, each of which is incorporated by reference.
Referring to FIGS. 1-3, the operation of the optical image-display system 10 is discussed.
Referring to FIG. 1, the image generator 12 starts scanning an image at an initial pixel location X=0, Y=0 and stops scanning the image at an end pixel location X=n, Y=m, where n is the number of pixels in the horizontal (X) dimension of the image and m is the number of pixels in the vertical (Y) dimension of the image. Specifically, the beam generator 16 modulates the intensity of the image beam 18 to form a first pixel Z0,0 of the scanned image when the reflector 22 directs the beam onto the location X=0, Y=0. As the reflector 22 sweeps the beam 18 toward the location X=n, Y=m, the generator 16 periodically modulates the intensity of the beam to sequentially form the remaining pixels of the image including the last pixel Zn,m. Then, the image generator 12 starts scanning the next image at the location X=0, Y=0, and repeats this procedure for all subsequent images.
Referring to FIG. 2, during the scanning of the image, the reflector 22 sinusoidally sweeps the beam 18 bi-directionally in the horizontal (X) dimension at a horizontal sweep frequency fh=1/th, where th is the period of the horizontal sinusoid. FIG. 2 is a plot of this horizontal sinusoid, which indicates the position of the beam 18 in the horizontal (X) dimension versus time, where + corresponds to the right side of the screen 14 and − corresponds to the left side. As this plot shows, the reflector 22 oscillates in a sinusoidal manner about the pivot arms 24a and 24b at fh, and thus sinusoidally sweeps the beam 18 from side to side of the screen 14 at the same frequency. The horizontal sweep is bi-directional because the beam 18 is “on”, and thus generates pixels, in both the left-to-right (+X) and right-to-left (−X) horizontal directions. Although not required, fh may substantially equal the resonant frequency of the reflector 22 about the arms 24a and 24b. One advantage of designing the reflector 22 such that it resonates at fh is that the scan assembly 20 can drive the reflector in the horizontal (X) dimension with relatively little power.
Referring to FIG. 3, the reflector 22 also linearly sweeps the beam 18 uni-directionally in the vertical (Y) dimension at a vertical sweep frequency fv=1/tv, where tv is the period of the vertical saw-tooth wave. FIG. 3 is a plot of this saw-tooth wave, which indicates the position of the beam 18 in the vertical (Y) dimension versus time, where + corresponds to the bottom of the screen 14 and − corresponds to the top. As this plot shows, during a vertical scan period V, the scan assembly 20 linearly rotates the reflector 22 about the pivot arms 26a and 26b from a top position to a bottom position, thus causing the reflector to sweep the beam 18 from the top pixel Z0,0 of the screen 14 to the bottom (pixel Zn,m) of the screen (−Y direction). During a fly-back period FB, the scan assembly 20 quickly (as compared to the scan period V) rotates the reflector 22 back to its top position (Z0,0) to begin the scanning of a new image. Consequently, tv=V+FB such that the vertical sweep frequency fv=1/(V+FB). Moreover, the vertical sweep is unidirectional because the beam 18 is “on” only during the scan period V while the reflector 22 sweeps the beam from top (Z0,0) to bottom (Zn,m) (−Y direction), and is off during the flyback period FB when the reflector 22 returns to its top position (Z0,0). One advantage of vertically sweeping the beam linearly and uni-directionally is that this is compatible with conventional video equipment that generates video images for display using this same vertical sweeping technique.
Unfortunately, uni-directionally sweeping the beam 18 in the vertical (Y) dimension may increase the cost, complexity, size, and power consumption of the system 10. Referring to FIG. 3, the vertical-sweep saw-tooth wave includes many harmonics of the fundamental vertical sweep frequency fv. For example, if fv=60 Hz, then the saw-tooth wave has significant harmonics up to approximately 3600 Hz (the 60th harmonic, i.e., 60×fv). The vibrations that these higher harmonics introduce to the reflector 22 may cause a significant error in the vertical (Y) location of the beam 18. That is, the reflector 22 may not rotate smoothly through the vertical scan, producing a vertical “jitter” or “ripple” that may cause the location where the beam 18 strikes the screen 14 to be misaligned with the location of the pixel Z that the beam is currently forming. One way to reduce or eliminate this error is to include a feedback loop (not shown in FIG. 1) in the scan assembly 20 to smoothen the rotation of the reflector 22 during the vertical scan period V. Such a feedback loop is disclosed in U.S. Pat. No. 6,140,979, which is incorporated by reference. Unfortunately, such a feedback loop often includes complex circuitry that can occupy significant layout area, and, thus, may increase the complexity, size, and cost of the image generator 12. Furthermore, quickly rotating the reflector 22 from its bottom position (Zn,m) to its top position (Z0,0) during the flyback period FB often requires that the scan assembly 20 drive the electromagnets (not shown) that rotate the reflector 22 with a significant peak current. Unfortunately, this may increase the power consumed by the image generator 12 and the size of the scan assembly's current-driver circuit (not shown), and thus may further increase the cost of the image generator.
Another way to reduce or eliminate the ripple error is to generate a drive signal that offsets the non-linearity of the vertical scan. A variety of approaches can be applied to reduce the ripple.
In one such approach, a feedback loop in the scan assembly 20 compares the detected angular position about the vertical axis with an idealized waveform. The loop then generates a drive signal to minimize the error, according to conventional feedback control approaches and smoothen the rotation of the reflector 22 during the vertical scan period V.
In another approach, a general analytical or empirical model of the vertical scan assembly is developed for the general characteristics of the scan assembly 20, using parameters of the a set of scan assemblies. Then, for the specific scan assembly 20 in use, the individual response is characterized during manufacture or at system start-up to refine the model parameters more precisely and the data representing the particular scan assembly 20 are stored in memory. The scan assembly then generates a drive signal according to the stored model to minimize the ripple.
In some cases, such feedback loops and adaptive control systems may include complex circuitry that occupies significant layout area or require specialized components, and thus may increase the complexity, size, and cost of the image generator 12.